This invention relates to a method and apparatus for storing and supplying fuel to chemical laser generators, more specifically, it relates to a cryogenic fuel supply system, particularly adapted for movable laser installations, which permits to store high quantities of gas required by the laser in containers having reduced volume and weight.
Chemical laser generators require for their operation large quantities of a gas (which will be called hereinafter xe2x80x9cprocess gasxe2x80x9d, for the sake of clarity), such as helium, which must be supplied at a temperature that is generally about normal ambient temperature, e.g. about 300 K (but also may have to be supplied at different, in particular lower, temperatures), and at medium pressure, e.g. in the order of 50 ata. However, the required large amounts of process gas create storage problems, particularly in mobile laser systems. To store the required quantities in containers of acceptable volume, the gas should be compressed to a relatively high density, e.g., in the order of hundreds of kilograms per cubic meter (e.g. 200 Kg/m3). Achieving such a density at normal ambient temperature requires that the gas be compressed to extremely high pressures, about 1500 ata, and correspondingly, storage containers are required which have thick walls. Containers of the required large size and with such thick walls, would have weights of several tons or even tens of tons, and making them would pose an engineering problem. Also, the compression of ethylene and NF3 is not practical nor cost effective, since the compression of NF3 creates safety problems and ethylene condenses during expansion to working pressures but these gases are used in smaller amounts than helium.
Further, helium, which is prevalently used as a process gas, and other process gases, must be supplied to the laser apparatus, when this latter is operated, under critically defined supply rate and pressure. Additionally, with present day technology, the temperature must not be lower than a certain limit. The required operating or work pressure is much lower than that required to compress helium to the desired density (e.g. it may be about 80 ata), and the expansion of helium from, e.g., 1500 ata to 80 ata would cause a drop of temperature to far below the acceptable laser operating pressure.
The laser generator, in all its parts, must be capable of being switched on, and therefore brought to the required operating or work conditions, in a very short time, not more than one half of a second. This fact creates engineering problems that do not exist in apparatus that works continuously or for which long switch-on times are acceptable. Therefore the process gas, no matter how it is stored, must be made available in the required work conditions in half a second or less.
It is therefore a purpose of this invention to provide an apparatus for supplying process gas to laser generators, which includes a system for storing the process gas comprising storage tanks the volume and the weight of which is greatly decreased, with respect to those of the prior art.
It is another purpose to provide such an apparatus in which the volume and the weight of the storage tanks for the process gases is smaller by at least an order of magnitude than in the storage systems of the prior art.
It is a further purpose to provide such an apparatus that supplies the process gas to the laser generator at the desired work conditions, particularly at the desired pressure and temperature.
It is a still further purpose to provide such an apparatus that permits the laser generator to be switched on in no more than one half of a second.
It is a still further purpose of this invention to provide such an apparatus, having the aforesaid advantages, and which can be applied to store helium, ethylene, NF3 and D2 
It is a still further purpose of this invention to provide such an apparatus which is suitable for movable installations.
It is a still further purpose of this invention to provide such an apparatus which does not require the compression of the process gas to the operating pressure.
It is a still further purpose of this invention to provide such an apparatus wherein the gas is stored in supercritical condition, at a density greater than that of the corresponding liquid.
It is a still further purpose of this invention to provide such an apparatus which is particularly suitable for the storage and rapid supply of a gas, e.g. such as DF Chemical Lasers or COIL in Airborne Laser.
It is a still further purpose of this invention to provide a process whereby process gases can be stored in a relatively small volume.
It is a still further purpose of this invention to provide a process for supplying process gases to the laser generator at the work temperature and pressure and at the required rate.
It is a still further purpose of this invention to provide such a process whereby the laser generator can be switched on in no more than one half of a second.
Other purposes and advantages of the invention will appear as the description proceeds.
The method, according to the invention, for the storage and delivery of process gases to the laser generator comprises the following steps:
a) cooling the process gas to a few degrees K, at the required work pressure, for example, to about 4 K at 80 ata, to reach a desired density, which is preferably in the order of hundreds of kg/m3;
b) storing the cooled gas in a cryogenic environment at said work pressure;
c) at the moment of use, gradually releasing the process gas from the cryogenic environment and concurrently heating it,
d) controlling the process gas flow rate from the cryogenic environment so as to obtain the required work flow rate; and
e) controlling the heat supplied to the gas so as to maintain it constantly at the required work pressure.
In an embodiment of the invention, the heating of the gas takes place preferably partly in the cryogenic space and partly outside it. The heating within the cryogenic space controls the gas flow rate and the heating outside it brings it substantially at the required work temperature. The word xe2x80x9csubstantiallyxe2x80x9d is used in this specification and claims to indicate that two temperatures do not differ from one another by more than 50xc2x0 C. If the temperature is considered not critical, at least within certain limits, is not critical, the second mentioned heating can, in some embodiments, be omitted. As the gas is released from the cryogenic space, its density decreases and its pressure would decrease, if its temperature remained constant. The pressure remains constant because the gas is heated, and therefore heating and release of the gas must be synchronically controlled so that their contrary influences on the pressure balance each other.
The heat for heating the process gas is preferably supplied to the heat exchangers by means of a heating gas. To achieve the aforesaid synchronic control of the heating and the release of the process gas, and in view of the fast response required from the system to provide a short switch-on time, the invention comprises carrying out the heat exchange between the process gas and the heating gas in a very rapid manner. From the apparatus viewpoint, this requires providing heat exchangers having small, ideally negligible, thermal mass, so as to accelerate the heat exchange and to allow fast response of the system. On the other hand, since such heat exchangers must have thin walls, they cannot withstand considerable pressure, and therefore the invention comprises feeding the heating gas at a pressure close to the work pressure of the process gas, e.g. about 80 ata.
To carry out the invention in said preferred way, the heating gas can be generated in various ways. It can result from the catalytic, exothermic decomposition of a solid or liquid substance, particularly, in a preferred embodiment, hydrogen peroxide, which produces H2O and O2, and, because of the decomposition heat, a mixture of steam and oxygen at elevated temperature, e.g. about 300xc2x0 C. Such a mixture can be used as a heating gas, but, in a preferred embodiment, the further heat is generated by the combustion of a fuel with the oxygen generated by the said decomposition. Substantially the same pressure as the work pressure of the process gas is maintained in all the stages of the production and use of the heating gas. By xe2x80x9csubstantially the same pressure as the work pressurexe2x80x9d is meant in this description and claims a pressure that differs from the work pressure by no more than xc2x110 ata. Said pressure may be created by means of the process gas, as will be explained hereinafter.
The apparatus according to the invention comprises:
I)xe2x80x94a cryogenic storage tank for storing process gas at cryogenic temperatures and at work pressure, e.g. 80-85 ata;
IIxe2x80x94means for feeding process gas to said tank at said temperature and pressure;
IIIxe2x80x94conduit means for releasing said process gas from said tank and delivering it to the laser generator; and
IVxe2x80x94heating means for heating said process gas as it is released, whereby to control its flow rate, pressure and temperature as required for the operation of the laser generator
Vxe2x80x94a flow valve for controlling the timing and the flow of the process gas to the laser generator; and
VIxe2x80x94optionally, a cryogenic refrigerator for keeping the stored gas at the desired cryogenic storage temperature.
Preferably, said heating means comprise:
a) a heating gas generator apparatus;
b) heat exchanger means, having low thermal mass, for causing fast heat transfer from said heating gas to the process gas as this latter is released from the cryogenic storage tank; and
c) pressurizing means for applying to said heating gas the same pressure as that of the process gas.
More preferably, the heat exchanger means comprise:
d) a first heat exchanger, comprising a conduit for causing the heating gas to flow through said cryogenic storage tank; and
e) a second heat exchanger between said cryogenic storage tank and the laser generator;
In a preferred embodiment of the invention, the heating(g gas generator apparatus comprises:
f) a liquid oxidant supply vessel;
g) a primary gas generator vessel comprising a catalyzer bed for generating a gaseous mixture from the decomposition of said liquid oxidant; and
h) a fuel supply.
In said embodiment, the means for applying to the heating gas the same pressure as that of the process gas may consist in leading a part of the process gas to the liquid oxidant supply vessel, optionally through a pump for compensating pressure losses, to deliver the liquid oxidant to the gas generator and maintain, throughout the heating gas generator apparatus and the heat exchange means, a pressure substantially equal to the work pressure of the process gas, e.g. about 80 ata.
When a single heat exchanger or a first and a second heat exchanger are provided, the single or first one is preferably constituted by the cryogenic storage tank and by a pipe system contained within it, immersed in the cryogenic gas, and through which the heating gas flows, and will be called hereinafter xe2x80x9cinner heat exchangerxe2x80x9d. By the expression xe2x80x9ccryogenic gasxe2x80x9d is meant herein a process gas kept at cryogenic temperatures. The second heat exchanger, when present, is constituted by a vessel through which the heating gas flows and which contains a pipe system through which flows the process gas issuing from the cryogenic storage tank.
The heating gas issuing from the heat exchange means may be directly ejected through a separate ejector, or be utilized as part of the first or ejecting stream in the main ejector that is always present in laser generator apparatus. The apparatus of the invention further may comprise an auxiliary container for containing sufficient process gas for switching on and operating the laser until the process gas from the cryogenic storage tank becomes available at the appropriate temperature and pressure.